Floating supports advantageously present a hull with substantially vertical longitudinal sides in order to optimize their oil storage capacities, and also to obtain better behavior in heavy sea. However a hull with vertical sides is particularly disadvantageous in terms of behavior relative to pack ice. Thus, in U.S. Pat. No. 4,102,288 and U.S. Pat. No. 4,571,125, floating supports are proposed that present, amongst other means, sides with profiles that are curved or inclined so as to enhance ice breaking in the manner that is known for a ship's bow having a stem that slopes relative to the horizontal.
In known manner, an oil production floating support including a releasable mooring system of anchor lines anchored to the sea bottom and of bottom-to-surface connection pipes comprises:                a mooring buoy for said anchor lines and bottom-to-surface connection pipes, said buoy preferably being an annular buoy; and        said mooring buoy being fastened under the hull of the floating support to a rotary device having a tower-shaped structure referred to as a “turret”, said turret co-operating with the hull within a cavity passing through the entire height of the hull of the floating support, said turret being rotatably mounted relative to said hull via at least one rolling or friction bearing, preferably a rolling bearing, so as to allow said floating support to turn about a substantially vertical axis ZZ′ of said turret and of said cavity without causing said mooring buoy to turn relative to the same vertical axis ZZ′; and        said bottom-to-surface connection pipes rise within the cavity to a coupling for a plurality of said pipes, said coupling being secured to the floating support level with the deck of the floating support, said coupling being ratably mounted so as to allow said floating support to turn without turning said coupling that is of the rotary joint coupling type.        
In the above-described prior art, the rolling bearing is located either level with the deck of the floating support, or else in the bottom portion under water, i.e. the bearing is immersed, or indeed a combination of the above two configurations may be used.
Embodiments in which the rolling bearing is located solely level with the deck are suitable only for floating supports of relatively small height, in particular less than 15 meters (m). With greater heights, for floating supports having a height lying in the range 20 m to 25 m, in particular, the horizontal force on the turret resulting from the floating support turning gives rise to the structure of the turret bending along its length, thereby mechanically stressing the top rolling bearing and thus mechanically endangering its reliability of operation. Furthermore, when the rolling bearing is underwater in the bottom portion of the turret, this immersion affects the operating reliability and the durability of said rolling bearing, and above all gives rise to difficulties in performing maintenance operations. On-site action requires the use of divers and of considerable technical means, and it is generally necessary to perform such operations in a protected zone, such as a fjord, or better still in a dry dock, after the FPSO has been disconnected. Thus, when an FPSO is intended to remain in position for several tens of years without any programmed maintenance disconnections in dry dock or in a protected site, that type of turret is not suitable.
Supports of the above-defined type are known from GB 2 291 389 and EP 0 259 072.
WO 94/15828 describes a system for quickly connecting and disconnecting a mooring buoy, in which the mooring buoy has a top portion that is connected to the bottom of the hull of the floating support, more precisely via a mooring cavity that extends annularly at the bottom end of a cavity passing through the entire height of the hull of the floating support with the bottom-to-surface connection pipes passing up therethrough. The mooring buoy also has a bottom portion to which there are moored the anchor lines and bottom portions of bottom-to-surface connection pipes extending to the sea bottom, said bottom portion of the mooring buoy being rotatably mounted by means of a rolling bearing that is completely immersed, enabling said bottom portion to turn relative to the top portion of the mooring buoy secured to the hull.
That type of system with a completely immersed rotary portion and rolling bearings that are completely immersed is not suitable for mooring bottom-to-surface connection pipes in large numbers, for which it is desirable to propose a system in which at least some of the rolling bearings are situated out of the water so that they can be maintained more easily and so that they can be implemented in operating conditions that are less constraining.
In WO 94/15828, provision is made at the bottom of the hull for internal tanks presenting a large area in horizontal section in which atmospheric pressure or preferably a vacuum is established. Said internal tanks present a large area of contact in horizontal section with the top portion of the mooring buoy, with the buoy being designed to be fastened thereagainst. For this purpose, an interstitial annular zone is created between the mooring buoy and the tank at atmospheric pressure at the bottom of the hull of the ship, which zone is defined by two concentric annular gaskets, which annular zone of small volume is put into contact with the chamber at atmospheric pressure at the bottom of the hull of the ship in order to create positive buoyancy for the assembly constituted by the mooring buoy and the anchor lines and the bottom-to-surface connection pipes that are pressed against said contact area.
Under certain conditions, it may be desirable to disconnect only the bottom-to-surface connection pipes, while maintaining said floating support moored to the bottom by said anchor lines, which anchor lines are moored on said mooring buoy.
Circumstances of this type occur in particular when it is desired temporarily to shelter the bottom-to-surface connection pipes since they are more fragile than the anchor lines, in particular when the bottom-to-surface connection pipes include flexible pipes providing the connection between said mooring buoy and a buoy immersed in the sub-surface, without it being necessary to cast off said anchor lines.
In mooring systems with a single annular mooring buoy fastened to a turret, as in GB-2-321 631, having fastened thereto both said anchor lines and said connection pipes, it is not possible to cast off the bottom-to-surface connection pipes without previously disconnecting and lowering the mooring buoys to which they are moored, whenever the sea is rough both at the surface and in the sub-surface. It is then necessary initially to cast off the mooring buoy in order to lower it to a sufficient depth where the sea is calmer and where there is no risk of damaging the bottom-to-surface connection pipes while they are being cast off from the buoy. Otherwise, there would a risk of damaging the connectors at the top ends of the portions of bottom-to-surface connection pipes that are connected to the buoy. However disconnecting the buoy then implies that the floating support is no longer anchored, at least temporarily, and that is not always desirable.
EP-0 831 023 describes a mooring buoy constituted by two independent portions consisting in an annular first buoy having moored thereto exclusively said anchor lines for the floating support, and a second buoy that is coaxially secured in releasable manner to said first buoy, i.e. it is disconnectable therefrom, occupying the central orifice of said first buoy, with said bottom-to-surface connection pipes being moored exclusively to said second buoy. Nevertheless, in the embodiment as described in EP-0 831 023, both buoys are completely contained in a cavity within the hull of the floating support and are thus connected and therefore disconnectable exclusively by mechanical locking means that are complicated and very difficult to implement when forces become large, which forces may reach or exceed 5000 (metric) tonnes (t) to 6000 t.